The present invention relates to an optical pickup device installed in an optical disk device for optically recording/reproducing information to/from an information recording medium such as an optical disk, or more specifically, relates to an optical pickup device capable of precisely recording/reproducing information to/from an optical disk having a plurality of recording/reproduction layers.
Recently, practical application of optical disks is promoted in audio, video, computer, and other various fields since an optical disk is capable of recording massive information signals at high density.
In compact disks (CD), video disks, mini disks (MD), computer-use magneto-optical disks, and the like which are now widely put on the market, a 1.2 mm thick substrate is usually used. An objective lens of an optical pickup is also usually designed so as to correct aberration which occurs due to the 1.2 mm thick substrate.
On the other hand, various techniques are examined to increase recording capacity, including a technique of improving optical resolution by increasing a numerical aperture (NA) of an objective lens, and a technique of improving a recording layer is multilaminate.
For example, the Japanese Publication for Laid-Open Patent Application No. 5-151609/1993 (Tokukaihei 5-151609) discloses an optical disk device for reproducing information from an optical disk having a plurality of data layers so that data recorded in the data layers are separately reproduced from the respective data layers. The recording/reproduction layers of the foregoing multilaminate disk are formed by alternately laminating transparent substrates and aerial layers. Information is recorded/reproduced by shifting a focus of an objective lens in an optical axis direction by driving it with an actuator.
In the foregoing example, return light from recording/reproduction layers adjacent to the target layer does not affect since the layers are disposed at sufficient distances. For example, a focus error signal (FES) of an n""th layer becomes 0 when an (n+1)""th or (nxe2x88x921)""th layer is brought into focus, thereby causing no affect such as offset on FESs of other layers. However, since the layers are disposed at great distances, in the case where focus servo is applied to each layer, a total thickness of the whole disk substrate to be brought into focus greatly varies. Therefore, it is also necessary to correct spherical aberration which is generated on each layer, by using an aberration compensator.
As a disk free from the foregoing problem, a double-layer disk having two data layers at a distance (for example, 40 xcexcm to 70 xcexcm) which is very small as compared with a thickness of the substrate has been proposed as a digital versatile disk (DVD) or the like. In this case, spherical aberration occurring due to a difference in the substrate thickness is sufficiently small, and hence no aberration compensator is needed.
However, as to a disk in which recording/reproduction layers are laminated at such a small distance, when a light beam accesses one recording/reproducing surface, a reflected light from the accessed recording/reproducing surface is affected by return light from other recording/reproducing surfaces adjacent to the accessed surface. Therefore, a focus error signal for focus adjustment of the light beam is also affected by the return light, and as a result, precise focus adjustment cannot be conducted.
An example of an optical system for use with the foregoing multilaminate disk in which layers are formed at sufficiently small distances is disclosed, for example, in the Japanese Publication for Laid-Open Patent Application No. 9-161282/1997 (Tokukaihei 9-161282). The optical system is arranged so that, as shown in FIG. 16, light from a semiconductor laser 1 is converged onto an optical disk 5 by an objective lens 4, and a returned light therefrom is led to a light receiving element 6 by a three-division hologram element 2. The semiconductor laser 1, the hologram element 2, and the light receiving element 6 are integrally provided.
As shown in FIG. 17, the light receiving element 6 has (i) two main light receiving sections 6a and 6b for focus error signal (FES) detection use, provided adjacent to each other, and (ii) sub light receiving sections 6e and 6f for focus error signal compensation use, provided outside the main light receiving sections 6a and 6b, respectively. Either the main light receiving section 6a or 6b receives return light, depending on the direction of the focus adjustment. The sub light receiving sections 6e and 6f are disposed at positions such that they detect light when, in a defocus state, the return light falls also outside the main light receiving sections 6a and 6b. Thus, by using a pair of beam spots formed with the focus-error-detection-use return light on the sub light receiving sections 6e and 6f, a focus error is detected and a focus error signal is generated.
More specifically, let output signals of the light receiving sections 6a, 6b, 6e, and 6f be Sa, Sb, Se, and Sf, respectively, then, the focus error signal FES can be computed by (Sa+Sf)xe2x88x92(Sb+Se). As a result, when the light beam is projected to outside the main light receiving section 6a (or 6b) as well in a defocus state due to a displacement beyond the dynamic range, an output signal of the sub light receiving section 6e (or 6f) becomes intense, whereby the focus error signal FES becomes weaker. Thus, the focus error signal FES is intensified as a displacement from the just focus position increases, and thereafter abruptly weakens when the displacement exceeds a certain level. Therefore, by appropriately setting the sizes and arrangement of the main light receiving sections 6a and 6b and the sub light receiving sections 6e and 6f, only the focus error signal obtained from the recording/reproducing surface scanned can be used as an effective signal, while influences of return light from the recording/reproduction layers adjacent to the scanned layer can be eliminated.
Thus, theoretically, a precise focus error signal is obtained by the foregoing optical pickup device, and therefore, a recording/reproducing operation can be carried out with precision.
In assembling the pickup device, however, an assembly error naturally exists. In the focus adjusting operation, such an error makes the change of shape of the beam spot formed with return light on the light receiving element 6 different from the normal change thereof, thereby causing compensation of the focus error signal to become excessive or insufficient. As a result, it is impossible to obtain an adequate FES curve in recording/reproducing information to/from an optical disk having a plurality of recording/reproduction layers.
Further, in the case where an assembly error occurs, the return light to light receiving sections 6c and 6d for radial error signal production use is projected thereon with displacement from an ideal position. Therefore, the light receiving sections 6c and 6d need to be formed to greater sizes with all errors taken into consideration, so that the return light never fails to fall within the light receiving sections 6c and 6d, even when the displacement is greatest. On the other hand, a signal frequency band of the light receiving element is required to be higher, as capacity of an optical disk such as a DVD increases. To cope with such a requirement, however, it is necessary to reduce a size of the light receiving element, and this is contradictory to the aforementioned requirement of enlarging the light receiving element.
The present invention was made in light with the above-described problems, and the object of the present invention is to provide an optical pickup device capable of performing precise recording/reproducing operations with respect to an optical disk having a plurality of recording/reproduction layers even with assembly errors produced in assembling the pickup device, by optimizing shapes of sub light receiving sections for focus error signal production use and shapes of light receiving sections for radial error signal production use.
To achieve the aforementioned object, a first optical pickup device of the present invention, in which a light beam emitted from a light source is converged onto an optical recording medium through an optical system, and a shift which a focal point of the light beam has made from the optical recording medium is detected based on return light returning from the optical recording medium through the optical system, is arranged so as to comprise (1) main light receiving means having at least two main light receiving sections, each main light receiving section producing a main signal in accordance with a quantity of the return light incident thereon, (2) sub light receiving means having sub light receiving sections, when the shift of the focal point is beyond a dynamic range and the return light becomes incident also on outside the main light receiving sections and partly on the sub light receiving sections, each sub light receiving means producing a sub signal in accordance with a quantity of the part of the return light that is incident thereon, and (3) error signal producing means for producing a focus error signal by compensating the main signal by using the sub signal, and the optical pickup device is arranged so that the sub light receiving means adjusts an offset of the focus error signal due to an arrangement error of an optical element constituting the optical system, by using the sub signal.
In the first optical pickup device, the error signal producing means corrects the main signal obtained from the main light receiving sections by using the sub signal obtained from the sub light receiving sections, whereby in a great defocus state, the focus error signal abruptly decreases when the shift is beyond the dynamic range. With this, focus control suitable for recording/reproduction with respect to an optical disk having a plurality of recording/reproduction layers can be performed. Moreover, since the sub light receiving means adjusts an error of the focus error signal due to an arrangement error (an assembly error or the like) of the optical element by using the sub signal, it is easy to suppress the offset of the focus error signal even in the case where the shape of the light beam spot varies due to the arrangement error.
The foregoing optical pickup device is preferably, for adjustment by using the sub signal, arranged so that (i) the sub light receiving section has an effective light receiving area smaller than that of the main light receiving section, and (ii) the sub light receiving means includes level adjusting means for adjusting a level of the sub signal. The above arrangement (i) allows the sub signal to be adjusted by using a simple structure, and ensures that the light receiving element including the main and sub light receiving sections is not hindered from having better signal characteristics in a high frequency band.
Furthermore, in the case where the effective light receiving area of the sub light receiving sections is optimally set not less than 25 percent and not more than 80 percent of the effective light receiving area of the sub light receiving sections, compensation of the focus error signal is prevented from becoming excessive or insufficient in a great defocus state, irrelevant to presence or absence of the arrangement error.
To achieve the aforementioned object of the present invention, a second optical pickup device of the present invention, in which a light beam emitted from a light source is made to pass through a hologram element and is converged onto an optical recording medium through an optical element, and a shift which a focal point of the light beam has made from the optical recording medium is detected based on return light which returns from the optical recording medium and is diffracted by the hologram element, is arranged so as to comprise (1) main light receiving means having at least two main light receiving sections, each main light receiving means producing a main signal in accordance with a quantity of the return light incident thereon, the return light having a cross section shape in accordance with the shift which the focal point of the light beam has made from the optical recording medium, (2) sub light receiving means having sub light receiving sections, when the shift of the focal point is beyond a dynamic range and the return light becomes incident also on outside the main light receiving sections and partly on the sub light receiving sections, each sub light receiving means producing a sub signal in accordance with a quantity of the part of the return light that is incident thereon, and (3) error signal producing means for producing a focus error signal by compensating the main signal by using the sub signal, and the optical pickup device is arranged so that a first order diffractive angle of the hologram element is set substantially equal to a minimum diffractive angle in such an angle range as causes a first order diffracted light resulting on diffraction of the light beam from the light source by the hologram element to reach outside of a range of incidence to the optical element.
Incidentally, that xe2x80x9ca first order diffractive angle is substantially equal to a minimum diffractive anglexe2x80x9d means that the first order diffractive angle includes a minimum diffractive angle to which an angle corresponding to the arrangement error is added so that the arrangement error is taken in consideration.
The second optical pickup device is thus arranged so that the first order diffractive angle of the hologram element is set to a small degree in such a range that an emitted first order diffracted light does not enter the optical system (lens system) including the optical element (a collimator lens, an objective lens, and the like). Under presence of an arrangement error (an assembly error or the like), the variation of the light beam spot shape of the return light formed on both the light receiving sections during a focus adjustment operation differs depending on the first order diffractive angle. Therefore, by setting the first order diffractive angle small as described above, such a difference in variation of the light beam spot shape can be made small. This prevents compensation of the focus error signal from becoming excessive or insufficient, whereby satisfactory focus error characteristics (an FES curve) can be obtained in recording/reproducing information to/from an optical disk having a plurality of recording/reproduction layers.
The second optical pickup device is preferably arranged so that the first order diffractive angle represented by xcex8 satisfies:
(D+r2+xcex94)/L1+r1/L2xe2x89xa6tanxcex8xe2x89xa6(D+r2)/L1+r1/L2
where D represents a distance between the light source and the main and sub light receiving sections, r1 represents a radius of the hologram element, r2 represents a radius of an optical element closest to the hologram element, L1 represents a distance between the optical element and the main and sub light receiving sections in a direction parallel with an optical axis of the hologram element, L2 represents a distance between the hologram element and the main and sub light receiving sections in a direction parallel with the optical axis of the hologram element, and xcex94 represents an arrangement error of the optical element.
To achieve the aforementioned object of the present invention, a third optical pickup device, in which a light beam emitted from a light source is converged onto an optical recording medium through an optical system, and a shift which a focal point of the light beam has made from the optical recording medium and a shift of the light beam in an radial direction on the optical recording medium are detected based on return light returning from the optical recording medium through the optical system, is arranged so as to comprise (1) focus control means including at least two focus-error-detection-use light receiving sections for producing a first signal in accordance with a quantity of the return light incident thereon, the focus control means producing, based on the first signal, a focus error signal in accordance with the shift of the focal point of the light beam, (2) radial control means having at least two radial-error-detection-use light receiving sections for providing a second signal in accordance with a quantity of the return light incident thereon, the radial-error-detection-use light receiving sections being provided so as to have the focus-error-detection-use light receiving sections provided therebetween, the radial control means producing, based on the second signal, a radial error signal in accordance with the shift of the light beam in the radial direction, and the optical pickup device is arranged so that (i) the radial-error-detection-use light receiving sections are provided at least in regions to which the return light shifted due to an arrangement error of an optical element constituting the optical system is incident, and (ii) the radial-error-detection-use light receiving sections have effective light receiving areas, respectively, which are substantially equal to each other.
In the third optical pickup device, return light is surely received even in the case where the return light to be incident on the light receiving sections for radial error signal detection use is shifted due to an arrangement error, and moreover, the effective light receiving area can be minimized. By so doing, a difference between the respective effective light receiving areas of the light receiving sections for focus error signal detection use and that for radial error signal detection use can be made smaller, whereby deterioration of frequency characteristics is prevented. As a result, the light receiving element including both the light receiving sections is not hindered from having better frequency characteristics in a high frequency band. Further, since the effective receiving areas of the light receiving sections for the radial error signal detection use are set substantially equal to each other, the light receiving sections have the same frequency characteristics. Therefore, in reproducing information from an optical recording medium having a plurality of recording/reproduction layers, occurrence of radial offset can be prevented even in the case where stray light from a layer adjacent to a layer accessed is incident on the light receiving sections for radial error signal detection use.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.